The present invention relates generally to x-ray tubes and, more particularly, to a method of manufacturing an x-ray tube bearing assembly having a coated bearing cage therein.
X-ray systems typically include an x-ray tube, a detector, and a bearing assembly to support the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, is located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation typically passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. One skilled in the art will recognize that the object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in a computed tomography (CT) package scanner.
X-ray tubes include a rotating anode structure for the purpose of distributing the heat generated at a focal spot. The anode is typically rotated by an induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. An x-ray tube cathode provides a focused electron beam that is accelerated across an anode-to-cathode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is necessary to rotate the anode assembly at high rotational speed. This places stringent demands on the bearing assembly, which includes tool steel ball bearings and tool steel raceways.
Bearings used in x-ray tubes are required to operate in a vacuum, which precludes lubricating with conventional wet bearing lubricants such as grease or oil. X-ray tube bearing rolling elements (i.e., bearing balls) are typically coated with a solid layer, or tribological system, of a metal with lubricating properties, such as silver, lead, or lead-tin. The lubricating metal that coats the bearing balls helps to reduce friction between adjacent balls and between the balls and the raceway. Despite the lubricating metal coating, however, a large amount of friction and heating is present at contact points between the balls and the raceway. The operating conditions in the x-ray tube environment, where temperatures in the vacuum environment range from 300-500 degrees Celsius and stress levels on the bearing balls can exceed 2.5 GPa, creates yet additional challenges for the bearing
Failure of a bearing in an x-ray tube is typically by wear of the plated silver and loss of the silver from a contact region between adjacent bearing balls and between the bearing balls and the raceway. Wear of the plated silver can occur because the balls in the bearing are not evenly spaced around the raceway and the ball-to-ball space positions are changed when the bearing is running. When a bearing ball is transitioned from a load zone to a non-load zone in the bearing, the ball rapidly moves out of the load zone and hits an adjacent ball due to load release. This load release results in a large impact load between adjacent bearing balls. The impact load damages a ball surface by causing indentations on the surface at the ball-to-ball contact point. Additionally, the rotation of adjacent bearing balls are opposite to one another. The rotational velocity of the ball surfaces, in inverse directions, creates high skidding torque and heat build-up when the adjacent balls contact one another. The high skidding velocities and internal heat created by ball-to-ball contact causes tremendous wear and lubrication damage so as to reduce bearing life. Thus, impact indentations, skidding wear, and heat build-up all serve to affect bearing performance and durability.
Therefore, it would be desirable to have a method and apparatus to eliminate the problems of skidding wear and dynamic impact load between adjacent balls in a bearing assembly. It would also be desirable to reduce skidding wear and heat build-up between such an apparatus and the bearing balls that may lead to premature failure of the x-ray tube.